Cronofy - Unified Calendar API for developers
TeamLink - TeamLink is one of the leading and sumptuous solutions that is making web and video meetings extremely easy and efficient.
Calendarix - Calendarix is a powerful and easy to use web-based calendar that runs on PHP and MySQL.
Google Duo - Google's answer to Facetime, high-quality video calling on iOS and Android.
Campaign Calendar - Campaign Calendar by Bracket Labs builds a marketing calendar from your Salesforce Campaigns.
Around - Find people to hang out with.